Template effects zeolite synthesis

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

The synthesis of zeolites is traditionally performed by crystallisation from a sol-gel mixture comprising reagents such as silica, sodium aluminate, sodium hydroxide and water. Another key component of the sol-gel mixture is a base whose main role is to regulate the pH of the mixture. If an organic base is used then a templating effect may also be observed... 

Lai et al. [30] prepared ZSM-5 zeolite membrane using a template (TPA+) free synthesis gel. In this way, calcinations, which often results in cracks in zeolite film, were unnecessary. Another advantage was that expensive and toxic templates were not applied in the synthesis gel, which made the preparation process more cost effective and environmentally friendly. 

Figure 5.4 The close fits of (left) the triquatemary alkylammonium cation, 2,3,4,5,6,7,8,9-octahydro-2,2,5,5,8,8-hexamethyl-lH-benzo[1.2-c 3,4-c 6-c"]tripyrrolium triquat , [(C4H4N(CH3)2)3] " within the pores of the zeolite ZSM-18 (Coordinates courtesy P.A. Cox) and (right) of the diquaternary cation (C7Hi3N-(CH2)4-NC7Hi3) within cages in the magnesioaluminophosphate STA-2 are good examples of the templating effect of organics in the synthesis of zeolites.

Template effects have been studied quite extensively in the synthesis of zeolites. They also play an important role in the formation of pore structures of... 

The use of ionic hquids (ILs) or eutectic mixtures as solvent in zeolite synthesis has been reported [178]. The IL, viz. l-methyl-3-ethylimidazolium bromide, was used both as solvent and as SDA. It solubilizes all components involved in the synthesis, while the electrostatic interaction between cations, IL and framework species form the basis of a strong templating effect [165, 178]. It was shown that the IL also works as a microwave absorber [172] and is suited for microwave-assisted zeolite synthesis. Because of the low vapor pressure of IL, the synthesis occurs at low pressure. The addition of amines to such synthesis mixtures yields pure API and ATV structures. The amine as well as the IL seem to exert a structure-directing effect [179]. This method is particularly useful for preparation of transition-metal functionalized frameworks, viz. Co-AlP04-n [180]. New zeotypes were obtained in the Al, P system, viz. SIZ-7, as well as in the Si system, viz. SIZ-12. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

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